Up to now, an optical disc for use as a recording medium in an information processing apparatus such as an optical disc drive is generally configured as shown in FIG. 1. An optical disc 11 shown in FIG. 1 includes a disc substrate 10 having a recording layer 12 formed on one plane thereof, and a transparent protection film 20 formed on the recording layer 12. The disc substrate 10 of the optical disc 11 has a center hole 14 formed at the center thereof. When the optical disc 11 is set up on a disc rotation drive unit provided on the optical disc drive, the center hole 14 engages with a centering member provided on the disc rotation drive unit, thereby the rotation center of the optical disc 11 corresponds to that of the disc rotation drive unit.
The recording layer 12 is formed on the optical disc 11 in various manners, for example, in such a manner that information pit sequences are formed along their concentrically shaped or spirally shaped recording tracks and thus formed information pit sequences are covered by a reflective film. For example, the thickness of the disc substrate 10 is around 1.1 mm and that of the transparent protection film 20 is around 0.1 mm.
Such optical disc 11 is set up on a turn table of the optical disc drive, not shown, the respective centers being brought together under centering, and is driven with a high-speed, while an optical beam L eradiated from an optical pickup comes into the recording layer 12 via the transparent protection film 20 so as to record information to the recording layer 12 or reproduce information recorded thereon.
It is necessary to favorably keep the flatness of the recording layer 12 of the optical disc so as to accurately record/reproduce information using the optical pickup in the condition that the optical disc is set up on the turn table of the optical disc drive and is rotated with a high-speed. In particular, to make the frequency of transmission signals higher so as to increase the recording density as well as the transfer rate of signals for recording/reproducing, it is necessary to suppress skew or leaning of the optical beam L coming into the optical disc against the optical axis, being raised by various kinds of factors. That is, deformation occurring in the radius direction of the optical disc is required to be prevented.
For example, according to the international publication WO99/00794, there is disclosed an optical disc in which a waterproofing film is formed on a certain plane thereof so as to prevent water from penetrating into the disc substrate, which can prevents deformation from occurring in the optical disc.
In addition to the water penetration, the influential factor of the deformation occurring in an optical disc is that, for example, one plane and the other plane of the optical disc 11 are of different layer structures and are made of different materials, thereby the disc substrate 10 is formed under the asymmetric structure along its thickness direction, as shown in FIG. 1.